ONLAY
PREPARATION
DEEPTHI P.R.
2nd YEAR MDS
MOD ONLAY
An indirect restoration, which is partly intracoronal and partly extracoronal that covers all the
cusps of a posterior tooth
Partly intracoronal, partly extracoronal: Cuspal protection- main feature
Modification of Cass II inlay
Utilizes intracoronal retention
Occlusal coverage: partial veneer extracoronal restoration
Inlay: Wedge- like retention
Outward pressure from the centre of the restoration: trying in, cementation & under occlusal
forces
Wedge like inlay increases risk of facture without protecting undermined cusps
No reinforcement of tooth structure
More protective restoration
Forces dissipated: non destructive to the tooth structure
Protection from occlusal forces: veneer of casting alloy over the occlusal surface
Craig et al. Superiority of MOD onlay in protecting teeth from stress
Ward (1926) , Mondelli et al. (1980)- isthmus width greater than one half the intercuspal
distance: restored using overlay
Reduction in premolar fracture
resistance- Mondelli et al
• Class I isthmus- 11% to 52%
• Proximo occlusal- 17% to 57%
• MOD- 36% to 61%
MOD inlays: cracks at 40 to 50 degrees apically from the corner of the cavity preparation
Onlays distribute forces over a wide area and reduce potential for breakage
Stress analysis – Farah et al. 1977
Stress producing
potential in an ordinary
inlay
Stress producing
potential in an inlay
with overextended
bevel
Stress producing
potential in an inlay
that is too wide
Stress analysis – Farah et al. 1977
The use of an onlay keeps stresses at a low level
Creates no hazard to the remaining tooth structure
INDICATIONS
Cuspal protection is to be considered- if the width of the lesion is 1/3 to ½ the intercuspal
distance
Cuspal protection is mandatory- width of the lesion exceeds ½ the intercuspal distance
Length: width ratio of the cusp is more than 1:1; but not exceeding 2:1- Cuspal protection is to
be considered
Length: width ratio of the cusp is more than 2:1- Cuspal protection is mandatory
When there is a need to change the dimension, shape and interrelationship of the occluding
tooth surfaces
Abutment teeth for RPD & fixed prostheses
Ideal supporting restoration for remaining tooth structure, combined with conservative tooth
involvement
When wear facets that exceed the cusp tips & triangular ridge crests facially/ lingually are to be
included
PREPARATION FOR CLASS I, II & SOME
CLASS III ALLOYS
GENERAL SHAPE
Dovetailed internally and follow cuspal anatomy externally
Proximally: Box shaped/ cone shaped
Capping of the functional and shoeing of the non- functional cusps
“Capping” refers to the complete coverage of the cusp/ cusps of a tooth with sufficient
extension of the bevel onto the buccal and lingual surfaces of the tooth to carry the margins of
the restoration into areas where stresses cannot be brought to bear directly into them*.
“ Shoeing” refers to a veneer coverage of the cusp of a tooth with only slight
finishing bevel on the crest of the cusp. The bevel is established either at right
angles to the long axis of the tooth or in a slight reverse direction
*MOD INLAY & ONLAY PREPARATIONS. WENKU.BAIDU.COM
PREPARATION FOR CLASS I, II & SOME
CLASS III ALLOYS
LOCATION OF MARGINS
Occluso-facio-lingual portion- Capped side
Facial & lingual margins: facial & lingual surface on the functional side where the cusps are
capped
Located gingivally enough to avoid contact with the opposing tooth in
centric & eccentric movements
Margins located so as to encompass all cuspal elements
¼ to 1/3 of facial & lingual surfaces
PREPARATION FOR CLASS I, II & SOME
CLASS III ALLOYS
Occluso-facio-lingual portion- Gingivally
Margins should include all facial/ lingual grooves
Parallel the contour of cusp tips & crests of marginal ridges
Increase in surface area & interlocking from uneven marginal termination –
aids retention
Conserves tooth structure
PREPARATION FOR CLASS I, II & SOME
CLASS III ALLOYS
Occluso-facio-lingual portion- Shoed side
Facial/ lingual margin will be located just gingival to the tip & ridge crests of the involved cusps &
away from the occlusal contact
Margins should follow the contour of the cusp tips & ridge crests
Proximal portion
Proximal margins will be in the exact location as inlays with modifications
Secondary flare given in all cases
PREPARATION FOR CLASS I, II & SOME
CLASS III ALLOYS
INTERNAL ANATOMY
Occluso- facio-lingual portion
Pulpal floor- as described for inlay, but deeper & at different levels
THE FACIAL/ LINGUAL WALL ON THE CAPPED SIDE:
Wall proper
Occlusal bevel
Table
Counterbevel
THE FACIAL/ LINGUAL WALL ON THE
CAPPED SIDE
Wall proper
Intracoronal portion of the wall
At least half of the vertical height of the wall
Completely in dentin
Slightly tapered from the opposing “wall proper” by 2- 5 degrees
Makes a definite angle with the pulpal floor
THE FACIAL/ LINGUAL WALL ON THE
CAPPED SIDE
Occlusal bevel
Intracoronal portion of the wall
At least 1/3 the total wall height
Hollow- ground long bevel following with a standard angulation paralleling the cusp’s occlusal
inclined planes
30 to 45 degrees from the long axis of the crown & approx. the same angulation as the wall
proper
Relieved at least 1 mm from the opposing cuspal elements
THE FACIAL/ LINGUAL WALL ON THE
CAPPED SIDE
Occlusal bevel
Two hollow ground bevels from the occlusally- one directed occluso mesially & one
occlusodistally following the direction of the occlusal inclined planes
Gives the bulk for the restorations at the inclined planes
Increased surface area of contact between the teeth & the cast help to
distribute forces physiologically on underlying tooth structures
Help to guide the cast with one specific relation with the preparation surface
Increase the immobilization of the restoration
THE FACIAL/ LINGUAL WALL ON THE
CAPPED SIDE
Table
Transitional area between the intracoronal & extracoronal parts of the
preparation
It can be partly in dentin/ enamel : relieved from opposing cusps by at least
1.5 mm in both static & functional contacts
Major resistance form: flat
Following cuspal anatomy in mesio- distal directions- 3 on each cusp:
increase surface area
provide more locking retention
conserve tooth structure
physiological distribution of occlusal loads
THE FACIAL/ LINGUAL WALL ON THE
CAPPED SIDE
Counterbevel
Extracoronal feature
Formed in areas of enamel & dentin- maybe completely in either
Relieved from opposing cuspal elements by at least 1 mm in both static & functional occlusal
contact
Hollow- ground, inclined away from the intracoronal parts gingivally- facially
or lingually
Each facial/ lingual inclined plane for a cusp
Each cusp on capped side: four different hollow- ground bevels
Embraces cuspal elements for retention &/ or support
Angulation of the counterbevel
Measured by the embracing angle: Angle between the bevel & the long axis of the crown- 30 to
70 degrees
Factors determining the angulation
Amount of indicated involvement for the facial/ lingual surface
Amount of needed retention
Type of cast alloy
Angulation of the counterbevel
Amount of indicated involvement for the facial/ lingual surface
The more the desirable involvement is, the less will be the embracing angle to preserve tooth
structure
Amount of needed retention
The shorter the preparation walls, the less will be the embracing angle
with the long axis
Maybe parallel to the intracoronal wall proper
Type of cast alloy
the less the castability, the greater will be the embracing angle
Allows more bulk marginally
Extent of the counterbevel
Least standardized in this part of the preparation
Additional determining factors other than those affecting the angulation:
Location of occluding contacts
Surface defects, grooves, pits, wear facets, decalcification
Extent of the counterbevel
Degree of needed embracing of the cuspal elements according to the future function of the
restoration
The need for changing the facial/ lingual contour of the tooth, especially its occlusal part
The need for retention, especially with shortened axial items in the preparation
The relation of preparation margins to the height of contour
THE FACIAL/ LINGUAL WALL ON THE
CAPPED SIDE
The junction between the four different parts of the capped side should be definite, but slightly
rounded
Improve reproducibility
Prevent stress concentration
FACIAL/ LINGUAL WALL ON THE SHOED
PART
Formed of three parts:
The “ wall proper”
The occlusal bevel
Similar to and reciprocating with
that on the capped side
The “ shoe”
Likened to table on the capped side, but relieved in lesser dimension
Most of the time- enamel
Functions similar as table
Shoe
When it ends in acute angled tooth structure, leaving frail enamel:
partial bevel- 4th plane
Bevel inclines away from the cavity preparation- facially or lingually- gingivally
following cuspal anatomy
Also given- when the shoe cannot properly continue as a proximal flare
Shoe avoided: for esthetic reasons, as its margins might terminate facially
Similar to the capped side: the junction between the components- definite, but rounded
PROXIMAL PORTION
Very similar to that of inlays
Wall proper & primary flare
Secondary flare with the flexible angulation is an integral feature
MODIFICATIONS FOR CLASS IV & SOME
CLASS III ALLOYS
Greater occlusal reduction to accommodate bulkier cast material
All circumferential tie features are hollow ground
All cusps capped, rather than shoed
The above modifications cause shortening of walls occluso-apically &
axio-proximally: maximum parallelism strived for
MODIFICATIONS FOR CLASS IV & SOME
CLASS III ALLOYS
Avoid any small/ complicated internal/ external details- boxes
Concavity of hollow ground bevels- include both enamel & dentin
Secondary flares: deeper & more extensive
MODIFICATIONS FOR CLASS V
MATERIALS
Cuspal protection: Capping only
Three distinct planes for the capped cusps:
- Exaggerated hollow- ground bevel extending from the
pulpal floor to the table
- table
- Exaggerated hollow- ground bevel extending from
the table to the facial/ lingual termination of the preparation
Increased gingival extension to improve retention
Embracing angle of the counter bevel more acute than that for the alloys
MODIFICATIONS FOR CLASS V
MATERIALS
More occlusal reduction for greater bulk of the material
Gingival, buccal & lingua walls of the proximal portion- very similar to preparation for cast
ceramic inlay
Usually deeper than preparations for Class I & II alloys , due to the frequent absence of boxedup internal portions
Parallelism strived for in all walls due to the loss of retention owing to the exaggerated hollow
ground bevels, increased occlusal reduction & limited dimensions of the proximal preparation
features
Avoid reverse secondary flares
MODIFICATIONS FOR CLASS V
MATERIALS
Extreme occlusal involvement bucco- lingually: three planes of a capped cusp- minimal selfresistance
Similar to Design 6 for Class II amalgam- cap is formed of two planes
Wall proper & flat table terminating in a right- angled cavosurface margin
The junctions between the walls, pulpal floor & table should be rounded
Facial/ lingual margin ends in middle/ occlusal 3rd of that surface or in dentin
Margins will not end in a concavity like a groove
The internal walls do not need cuspal bracing for retention
Remaining cuspal elements will not need reinforcing
Steps in the
Preparation of
an MOD Onlay
Planar Occlusal Reduction- Round end
tapered diamond & NO. 171 Bur
Planar Occlusal Reduction
Depth- orientation grooves on the occlusal surface with the round-end tapered diamond
One along the crest of each triangular ridge & one in each major developmental groove
Planar Occlusal Reduction
Occlusal reduction & depth orientation grooves- 1.5mm deep on functional cusp & 1mm deep
over most of the nonfunctional cusp
Maxillary tooth- nonfunctional facial cusp highly visible & care should be taken not to overcut
the facio-occlusal extension
Depth of the orientation grooves & the occlusal reduction itself – approximately 0.5mm in depth
at the facio- occlusal line angle
Planar Occlusal Reduction
Remove the tooth structure remaining between the depth orientation grooves with the roundend tapered diamond
Reduction should follow the original contours of the cusp, reproducing the basic geometric
inclined planes to the occlusal surface in the process
Corrugated multiplanar design:
-Creates space for a uniform bulk of metal
- Adds more strength to the restoration
Functional Cusp Bevel: Round end
tapered diamond & No. 171 bur
Functional Cusp Bevel
Depth- orientation cuts are made on the outward facing
inclines of the functional cusp
1.5mm deep at the cusp tip & fade out along the line that will
location of the occlusal shoulder
later mark the
Functional Cusp Bevel
Reduction is completed by removal of tooth structure
between the grooves
The bevel approximates the angle of the cuspal inclines in the
opposing arch
Extends around the central groove on the mesial & distal surfaces
of the tooth
Final extension can be delayed until the proximal boxes
have been instrumented
Functional Cusp Bevel
The planes of the occlusal reduction & functional cusp bevel: smoothed with No. 171 tapered
fissure bur
Inclined planes: well- defines, but with no sharp line or point angles
Smooth surface of occlusal reduction: removes defects that might
later interfere with the complete seating of the restoration
Check for Occlusal reduction
Visual inspection- important
Limited to the facial half o the occlusal surface
Lingual cusps: verified with utility wax/ thickness
gauge
Occlusal shoulder: No.171 Bur
Occlusal shoulder
Cut using a No. 171 bur, following the termination line of the functional cusp bevel
On the axial surface of the functional cusp
1mm wide shoulder, extends from the central groove on one
proximal surface to the central groove on the other proximal
surface
Provides space for a bulk of metal to reinforce the occlusal margin
on the functional cusp
Occlusal shoulder
Chamfer/ occlusal shoulder: occlusal finish
line
Ensures acute edge in the restoration margin,
with a nearby bulk of metal
Shoulder with bevel: easier to prepare
properly: advised for a novice
Isthmus: No.170 bur
Isthmus
If there was no existing restoration to be removed: isthmus is made at this point
1mm shallower than the isthmus of inlay; occlusal surface already
reduced
Smooth walled opposing facial & lingual walls with minimum taper
Provides 1/5 of the retention & a great deal of resistance
Proximal box: No. 169L & No. 170 burs
Proximal box
Preparation begun: No. 170 bur
Relatively intact proximal surface: 169L bur- smaller &
more easily kept away from the adjacent proximal surface
Barely break contact with the adjacent tooth : Mesial surface
1mm wide gingival floor
169L bur: accentuate the facioaxial & linguoaxial line angles
Proximal box
Repeat the process with the distal box
The facial extensions can be less conservative
Proximal box
Facial & lingual walls, line angles: 169 bur
Very little hand instrumentation of the angles needed
Proximal box
Bur leaned slightly to the center of the tooth & to the facial / lingual side- form the walls of the
boxes
Ensures occlusal divergence of the facial & lingual walls,
occlusal convergence of the axial walls
Proximal box
Width of the proximal contact areas with the adjacent tooth checked
Proximal flares started only once the boxes are finished
Proximal box
1mm wide enamel chisel; Binangle chisel/ hatchet: pane the facial & lingual walls
Walls: perpendicular to the direction of oblique/ rotating forces: resistance to the restoration
Planing Horizontal Surfaces: No. 957 bur
Planing Horizontal Surfaces
Smooth the pulpal floor of the isthmus that
joins the proximal boxes
Occlusal shoulder of the functional cusp bevel
1mm wide
Planing Horizontal Surfaces
Gingival floors of the proximal boxes are planed the last
Improves the resistance of the finished onlay to displacement when compressive occlusal forces
occur on the restoration
Proximal Flares: Flame diamond & Flame
bur
Proximal Flares
Started with tip of a flame shaped diamond from within the box
Allows the instrument to be inserted into the restricted embrasure space without injuring the
adjacent tooth
Proximal Flares
Wide enamel hatchet : 1.5 to 2 mm used to
shape the mesiofacial flare where esthetic
considerations are important
Sandpaper disk
Sharp instrument: smooth, unmarred flare &
finish line
Under rubber dam isolation
Careful: soft tissues
Gingival bevel: Flame diamond & Flame
bur
Gingival bevel
Flame shaped diamond: 0.5 to 0.7 mm wide bevel
along the entire gingival floor
Bend into the flares on the facial & lingual walls of the box
without any undercut
Lean the diamond over into the proximal box: avoids excessively long/ obtuse bevel
Rounding of the proximo- occlusal in the angle: acceptable
Gingival bevel
Flares & bevels: reinstrumented with the flame shaped carbide finishing bur
Smooth the flares & bevels
Produce a sharp, distinct finish line: facilitate the fabrication of a well- fitting restoration
Facial & Lingual bevels: Flame diamond &
No. 170 bur
Facial & Lingual bevels
Occlusal finishing bevel: facial cusp with No. 170 bur
Perpendicular to the long axis of the tooth: approx. 0.5 mm wide
If bur leaned over to the faciogingival: wider &
compromises esthetics
Facial & Lingual bevels
Round the bevel over onto the facial flares
Outer edge of the occlusal bevel, continuous
with the outer edge of the facial flare: smooth,
unbroken finish line in the transitional area
Sharp angle: Negative angle in the wax
pattern- unfinishable gap in the casting margin
Angle between the occlusal reduction & the
flares also rounded over
Avoid sharp projections- later interfere with
the complete seating of the final restoration
Facial & Lingual bevels
0.5mm bevel on the occlusal shoulder: blend
smoothly with the lingual flares at their
junction
Not extended too far gingivally
Wide & thin wax pattern- Incomplete casting
Angle between the functional cusp bevel &
the flares also rounded
Completed MOD Onlay Preparation on
Maxillary Premolar
MODIFICATIONS IN ONLAY TOOTH
PREPARATIONS
Facial/ lingual surface groove extension
Inclusion of Portions of the Facial and Lingual Smooth Surfaces Affected by Caries, Fractured
Cusps, or Other Injury
Enhancement of Resistance and Retention Forms
Modifications for Esthetics on Maxillary Premolars and First Molars
Endodontically Treated Teeth
Restoring the Occlusal Plane of a Tilted Molar
Facial or Lingual Surface Groove
Extension
A facial surface fissure (mandibular molar) / lingual surface fissure (maxillary molar) : similarly as
in inlay preparation.
This extension is sometimes indicated to provide additional retention form even though the
groove is not faulty.
Inclusion of Portions of the Facial and Lingual
Smooth Surfaces Affected by Caries, Fractured
Cusps, or Other Injury.
Shallow to moderate lesions on facial surfaces are included similar to inlay preparation
Fractured mesiolingual cusp of a mandibular molar
Use a No. 271 carbide bur to cut a shoulder perpendicular to
occlusal force by extending the proximal gingival floor
(adjacent to the fracture) to include the affected surface.
Shoulder partially provides the desired resistance form by being perpendicular to gingivally
directed occlusal force.
Use this instrument also to create a vertical wall in the remaining lingual enamel
Inclusion of Portions of the Facial and Lingual
Smooth Surfaces Affected by Caries, Fractured
Cusps, or Other Injury.
Short clinical crown: proximal grooves for additional retention with the No. 169L bur.
Linguogingival and lingual margins are beveled with the flame-shaped, fine-grit diamond
instrument to provide:
30-degree metal at the gingival margin
These two bevels should blend together
40-degree metal along the lingual margin
Enhancement of Resistance and
Retention Forms
(1) improve retention form
(2) resist forces normally opposed by the missing mesiolingual wall
(3) help protect the restored tooth from further fracture injury
Grossly weakened by caries or previous filling material prone to fracture under occlusal loads 
Cusp capping augmented by skirts, collars, or facial (lingual) surface groove extensions
Such onlays distribute occlusal forces over most or all of the tooth
Enhancement of Resistance and
Retention Forms
The lingual "skirt" / "collar" preparation/ the lingual surface groove extension on a maxillary
molar protect the facial cusps from fracture
The facial skirt extension(s)/ the facial collar preparation/ the facial surface groove extension on
a mandibular molar protect the lingual cusp(s) from fracture
Skirt Preparation
Thin extensions of the facial or lingual proximal margins of the cast metal onlay that extend from
the primary flare to a termination just past the transitional line angle of the tooth.
A skirt extension is a conservative method of improving both the retention & resistance forms
Relatively atraumatic to the health of the tooth: removes very little (if any) dentin
Usually the skirt extensions are prepared entirely in enamel
Lingual wall missing: skirt extension on the facial wall
Skirt Preparation
Facial wall missing: skirt extension of the lingual surface
When both the lingual and facial walls of a proximal boxing are inadequate: skirt extensions on
both the respective lingual & facial margins
The addition of properly prepared skirts to three of four line angles of the tooth virtually
eliminates the chance of postrestorative fracture of the tooth because the skirting onlay is
primarily an extracoronal restoration that encompasses and braces the tooth against forces that
might otherwise split the tooth
The skirting onlay is often used successfully for many teeth that exhibit the split-tooth
syndrome.
Other indications- Skirt
The proximal surface contour & contact are to be extended more than the normal dimension to
develop a proximal contact.
When improving the occlusal plane of a mesially tilted molar by a cusp capping onlay, reshaping
the mesial surface to a satisfactory contour and contact
Splinting posterior teeth together with onlays
Ease of soldering the connector(s) and finishing of the proximal margins is increased
Disadvantage: Increases the display of metal on the facial & lingual surfaces - avoided on the
mesiofacial margin of maxillary premolars and first molars
Preparation of Skirt
Slender, flame-shaped, fine-grit diamond instrument
Follows the completion of the proximal gingival bevel & primary flares
At the same time that the gingival bevel is placed, working from the lingual toward the facial or
vice versa
The long axis of the instrument parallel to the line of draw
The rotating instrument into the tooth to create a definite vertical margin, just beyond the line
angle of the tooth, providing at the same time a 140-degree cavosurface enamel angle (40degree metal angle)
Preparation of Skirt
Extending into the gingival third of the anatomic crown is usually necessary for effective
resistance form
The gingival margin of the skirt extension is occlusal to the position of the gingival bevel of the
proximal box
Preparation of Skirt
Less than one half of the tip diameter of the flame shaped diamond used top avoid creating a
ledge at the gingival margin
Blending with the primary flare & proximal gingival margin: Translate instrument from entrance
cut to proximal surface
Preparation of Skirt
Be sure not to overreduce the line angle of the tooth when preparing skirt extensions
The bracing effect of the skirt will be diminished
Holding the diamond instrument at the same angle that was used for preparing the
counterbevel, round the junction between the skirt and the counterbevel
Slightly round any sharp angles that remain after preparation of the skirt to avoid difficulties in
subsequent steps of completing the restoration.
Collar Preparation
To increase the retention and resistance forms when preparing a weakened tooth for a MOD
onlay capping all cusps, a facial or lingual "collar," or both, may be provided
No. 271 carbide bur at high speed parallel to the line of draw to prepare a 0.8-mm deep
shoulder around the lingual (or facial) surface to provide for a collar about 2 to 3 mm high
occlusogingivally (see Fig. 20-31, A and B).
Collar Preparation
To provide for a uniform thickness of metal, the occlusal 1 mm of this reduction should be
prepared to follow the original contour of the tooth & should round any undesirable sharp line
angle formed by the union of the prepared lingual and occlusal surfaces.
Lightly bevel the gingival margin of the shoulder with the flame-shaped,
fine-grit diamond instrument to result in a 30-degree metal angle at the
margin
Slot Preparation
I. Mandibular second molar that has no molar posterior to it and requires a MO onlay
restoration capping all of the cusps
After cusp reduction, the vertical walls of the occlusal step
portion of the preparation have been reduced so as to provide
very little retention form.
Distal slot:
(1) more conserving of tooth structure and of strength of the
tooth crown
(2) the linear extent of marginal outline is less
Slot Preparation
Use a No. 169L carbide bur whose long axis should parallel the line of
draw
The slot is cut in dentin so that if it were to be extended gingivally,
it would pass midway between the pulp and the DEJ so as to avoid:
(1) exposure of the pulp
(2) removal of the dentin supporting the distal enamel
(3) perforation of the distal surface of the tooth at the gingival termination of the
slot
Slot Preparation
Approx. dimensions of slot
(1) mesiodistally, the width (diameter) of the bur
(2) faciolingually, 2 mm and
(3) depth, 2 mm gingival of the normally positioned pulpal wall.
To be effective, the mesial wall of the slot must be in sound dentin
Slot Preparation
II. Maxillary first premolar requires a DO onlay restoration capping the cusps
Mesially positioned 1.5mm wide faciolingually slot
The mesial occlusal marginal outline in this preparation should be distal of the height of the
mesial marginal ridge
Modifications for Esthetics on Maxillary
Premolars and First Molars
On the facial cusps of maxillary premolars & on the mesiofacial cusp of the maxillary first molar,
the occlusal reduction should only be 0.75 to 1 mm on the facial cusp ridge to decrease the
display of metal
Increase progressively to 1.5 mm toward the center of the tooth to help provide rigidity to the
capping metal.
These cusps do not receive a counterbevel but are "stubbed" or blunted by the application of a
sandpaper disc or the fine-grit diamond instrument held at a right angle to the facial surface
The surface created by this blunting should be approximately 0.5 mm in width.
Modifications for Esthetics on Maxillary
Premolars and First Molars
The mesiofacial margin is minimally extended facially of contact to such a position that the
margin is barely visible from a facial viewing:
Secondary flare is omitted,
Wall and margin are developed with a chisel or enamel hatchet
Final smoothing with the fine grit paper disc is recommended when access permits.
The cavosurface margin should result in a gold angle of 40 to 50 degrees, if possible.
When more than ideal extension of the mesiofacial margin is necessary: place a composite
insert at this margin
When preparing the mesiofacial margin, no attempt is made to develop a straight mesiofacial
wall past the point of ideal extension
After caries excavation a glass-ionomer cement base is inserted to temporarily form the missing
portion of the wall
The cement is contoured to ideal form, and the preparation continue, terminating the
mesiofacial onlay margin in ideal position in the cement
Following cementation, remove (with small round burs) the glass-ionomer cement to a depth of
1 mm for a composite insert. Small undercuts should be prepared in the wall formed by the cast
metal onlay
After beveling enamel cavosurface, preparing a gingival retention groove
where, and if, enamel is thin or missing, insert the composite veneer
Endodontically Treated Teeth
Facial & lingual surfaces of an endodontically treated tooth are sound, it is more conservative
for the health of the facial and lingual gingival tissues not to prepare the tooth for a full crown
but for a MOD onlay future tooth fracture
Such features include skirt extensions and collar preparations
Onlay more of an extracoronal restoration that encompasses the tooth, such that the tooth is
better able to resist lateral forces that might otherwise fracture the tooth
Pulp chamber should be excavated to the chamber floor and sometimes into the canals (1 to 2
mm), & an amalgam foundation placed
Endodontically treated premolar for an onlay, the canal is usually prepared for a metal post
Tooth resist forces that might otherwise cause a horizontal fracture of the entire tooth crown
from the root
The post should extend roughly two thirds the length of the root & should terminate, leaving at
least 3 mm of the root canal filling material at the apical portion of the root.
Restoring the Occlusal Plane of a Tilted
Molar
When the unprepared occlusal surface (mesial portion) is less than the desired occlusal plane, a
corresponding decrease in occlusal surface reduction
The counterbevels on the latter surfaces often should be extended gingivally more than usual
When the unprepared occlusal surface (mesial portion) is less than the desired occlusal plane, a
corresponding decrease in occlusal surface reduction is indicated.
Often the mesiofacial and mesiolingual margins (on the "submerged" proximal surface) should
be well extended onto the respective facial and lingual surfaces to help in recontouring the
mesial surface to desirable proximal surface contour and contact
Minimal loss of tooth structure by preparing facial and lingual skirt extensions on the respective
proximal margins
CONCLUSION
Onlays: viable treatment option
Overcomes many shortcomings of inlay restorations
Improved resistance form: bracing tooth structure
Relevant in the occlusion- centred approach of restorative dentistry rather than the one which is
solely tooth- oriented
Prudent use : Valuable alternative for full coverage restorations in many situations
Thank you..